1. Field of the Invention
The present invention relates to electric submergible pumping systems for recovering liquids from a wellbore and, more particularly, to a submergible centrifugal pump adapted to be positioned in a wellbore in an inverted position to pump fluids downhole.
2. Description of Related Art
Electric submergible pumping systems are commonly used to recover liquids from subterranean wellbores, and generally comprise an electric motor that operates a multistage centrifugal pump. The usual configuration is for the pump to be placed in the wellbore above or "uphole" of the motor, with the pump's impellers being rotated so as to move the fluids upwardly, i.e. uphole. There are applications where the pump is inverted in the wellbore so that the pump is below or "downhole" of the motor, and the fluids are moved downwardly, i.e. downhole, such as in a fluid reinjection application. This inverted arrangement can cause problems such as premature bearing wear, excessive thrust washer wear, and pump failure.
The inverted arrangement causes problems because the typical centrifugal pump is designed to move fluids upwardly, so in a typical arrangement the weight of and the downthrust from the pump's impellers are supported by a relatively large thrust bearing located within the motor protector, which is positioned between the pump and the electric motor, as is well known to those skilled in the art. The thrust bearing in the motor protector is lubricated by a secondary fluid that is isolated from the wellbore fluids and that has greater lubricating qualities, so that the thrust bearing can carry greater loads than a bearing lubricated by the wellbore fluids. When the pump is inverted the weight of the impellers and downthrust cannot be carded by the protector's thrust bearing without rigidly linking the pump shaft and the protector shaft together. Rigid shaft connections have been used, but such connections can be very difficult to properly install with the correct tolerances in remote field locations.
When the pump's shaft and the protector's shaft are not rigidly connected, then in a conventional pump the weight of the impellers will be distributed over the relatively very thin upthrust washers on each impeller. When the motor is activated and the pump is rotated, the weight load on these thin upthrust washers will exceed their design load capability, which results in destruction of these washers. Thereafter, metal-to-metal contact between the rotating impellers and the stationary diffusers occurs which destroys the pump.
Further, when a pump is inverted the downthrust is reduced which causes the pump to be operated below its best efficiency point. In a typical pump installation, downthrust is comprised of an unbalanced hydraulic force and the weight of the impellers. When this typical pump is operated, the downthrust and impeller weight are collinear. Thus, the pump is designed so that the downthrust is approximately zero at the best efficiency point of the pump. Inverting the pump causes a reduction of the downthrust because the impeller weight is now acting opposite to the downthrust. This means that when the inverted pump is operating at its best efficiency point the impellers will be in upthrust and the effective operating range of the pump will be reduced unless an additional bearing is used to carry this extra upthrust.
To handle this extra upthrust from an inverted pump an additional motor protector bearing has been attached to the shaft of the pump on its fluid outlet end. This extra protector is a costly addition to this problem. There is a need for an inexpensive upthrust bearing that can be easily incorporated into a conventional centrifugal pump.